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Scheduling Introduction to Scheduling

Scheduling Introduction to Scheduling. Bursts of CPU usage alternate with periods of I/O wait a CPU-bound process an I/O bound process. When to Schedule. 1. At process creation 2. At process termination 3. At blocking system calls 4. At I/O interrupt

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Scheduling Introduction to Scheduling

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  1. SchedulingIntroduction to Scheduling • Bursts of CPU usage alternate with periods of I/O wait • a CPU-bound process • an I/O bound process

  2. When to Schedule 1. At process creation 2. At process termination 3. At blocking system calls 4. At I/O interrupt Preemtive and non-preemtive scheduling algorithms

  3. Goals of Scheduling

  4. Scheduling in Batch Systems (1) (a) First-come-first-served (FCFS) (b) Shortest-job-first (SJF) - provably optimal if all jobs are known in advance (c) Shortest-remaining-time-next (SRTN)

  5. Scheduling in Batch Systems (2) Three level scheduling

  6. Scheduling in Interactive Systems (1) • Round Robin Scheduling (a) list of runnable processes (b) list of runnable processes after B uses up its quantum Length of quantum: • too short=> waste of CPU time • too long => poor response

  7. Scheduling in Interactive Systems (2) A scheduling algorithm with four priority classes Static and dynamic priority allocations, e.g. 1/f.

  8. Scheduling in Interactive Systems (3) • Multiple queues • Shortest process next; aging (weighted average) • Guaranteed scheduling (1/n) • Lottery scheduling - biased randomness, passing tickets • Fair share scheduling

  9. Scheduling in Real-Time Systems Schedulable real-time system • Given • m periodic events • event i occurs within period Pi and requires Ci seconds • Then the load can only be handled if

  10. Policy versus Mechanism • Separate what is allowed to be done with how it is done • a process knows which of its children threads are important and need priority • Scheduling algorithm parameterized • mechanism in the kernel • Parameters filled in by user processes • policy set by user process

  11. Thread Scheduling (1) Possible scheduling of user-level threads • 50-msec process quantum, threads run 5 msec/CPU burst Application-specific thread schedulers

  12. Thread Scheduling (2) Possible scheduling of kernel-level threads • 50-msec process quantum • threads run 5 msec/CPU burst

  13. UNIX Scheduler The UNIX scheduler is based on a multilevel queue structure

  14. Scheduling in Windows (1) Mapping of Win32 priorities to Windows 2000 priorities

  15. Scheduling in Windows (2) Windows 2000 supports 32 priorities for threads

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